Consolidate FwLiteWeb data paths behind a single BaseDataDir

Problem

FwLiteWeb configures its on-disk locations as independent settings — LcmCrdt:ProjectPath and Auth:CacheFileName are set separately by callers (e.g. the Platform.Bible extension's launchFwLite in platform.bible-extension/src/main.ts). Nothing ties them together, so a caller can point them at different locations without anything catching it. #2417 added EnsureDataDirectoriesExist to FwLiteWebServer.cs to create both directories at startup, but that's a workaround for the missing-directory symptom, not the underlying inconsistency.

FwLiteMauiKernel already solves this with a single BaseDataDir (FwLiteMauiConfig.cs), from which ProjectPath, AuthCacheFilePath, FailedSyncOutputPath, LocalResourceCachePath, and log paths are all derived. FwLiteWeb should follow the same pattern.

Raised as a non-blocking suggestion on #2417: https://github.com/sillsdev/languageforge-lexbox/pull/2417#pullrequestreview-4641535206

Proposed change

• Add a BaseDataDir-style setting to FwLiteWeb's config (new FwLiteWebConfig or similar), mirroring FwLiteMauiConfig.
• Derive LcmCrdtConfig.ProjectPath, AuthConfig.CacheFileName, and (if applicable) CrdtConfig.FailedSyncOutputPath from it, instead of each being set independently.
• Reduce EnsureDataDirectoriesExist back to a single Directory.CreateDirectory(baseDataPath) call.
• Update platform.bible-extension/src/main.ts (launchFwLite) to pass one --FwLiteWeb:BaseDataDir=<dir> flag instead of the two separate LcmCrdt:ProjectPath / Auth:CacheFileName flags.

Out of scope / watch for

• Must not break appsettings.sdk.json's relative-path default (ProjectPath: "./fw-lite-projects") for local/dev SDK mode.
• Check for any other consumers of FwLiteWeb besides the Platform.Bible extension that set LcmCrdt:ProjectPath / Auth:CacheFileName directly, to avoid a silent breaking change.

Compare FwLiteMauiConfig.cs with the FwLiteWeb configuration and FwLiteWebServer.cs, then inspect appsettings.sdk.json and launchFwLite in platform.bible-extension/src/main.ts. Done means FwLiteWeb derives its project, auth cache, and applicable failed-sync paths from one BaseDataDir, creates that base directory, preserves the relative SDK default, and no other direct consumers are missed.